Production of high temperature articles and alloys therefor



1956 w. BETTERIDGE ETAL PRODUCTION OF HIGH TEMPERATURE ARTICLES ANDALLOYS THEREFOR Filed Dec. 2. 1952 2 Sheets-Sheet 1 l l I 700 800 900I000 [/00 I Inventors WALTER BETTSQ/DGE 4277/U1Q W FPA/V/(L/A/ yAttorney Oct. 9, 1956 W. BETTERIDGE EI'AL Filed Dec. 2, 1952 2Sheets-Sheet 2 Rupture Li/e hows J I Hg 700 800 90976 I000 //00 I 0 C O[0 a/00./57;c

\L moo H Fly. 6. ob K Inventors y C. M W ttorne y I PRGDUCTION OF HIGHTEMPERATURE ARTICLES ANE ALLOYS THEREFOR Walter Betteridge, Solihull,and Arthur William Franklin,

Quinton, Birmingham, England, assignors to The International NickelCompany, Inc, New York, N. Y., a corporation of Delaware ApplicationDecember 2., 1952, Serial No. 323,568

6 Claims. (Cl. 148-219) The present invention relates to heat-resistantnickel alloys and the heat treatment thereof, and, more particularly, toa special heat treatment for selected carboncontaining, age-hardenable,nickel-chromium alloys suitable for use under high loads at elevatedtemperatures.

In recent years, many alloys have been proposed for use at elevatedtemperatures, particularly heat-resistant and corrosion-resistant,nickel-base alloys of the agehardenable type. In employing these alloysin high temperature applications involving high stresses, it is usuallynecessary to heat treat the alloys in order to obtain the requiredcombination of high-temperature physical properties and also in order toensure satisfactory performance at elevated temperatures. Alloys fromwhich articles and parts, which are subjected to prolonged stress athigh temperatures, i. e., those in the order of 600 C. and upwards, aremade must possess not only resistance to corrosion at high temperaturebut also resistance to creep. The alloys most successfully used for thispurpose are nickel-chromium alloys which contain a precipitable phaseconsisting of nickel, titanium and aluminium and having the apparentcomposition Ni3(TiAl), i. e., the NisAl phase with partial substitutionof titanium for aluminium. This constituent enters into solid solutionat a high temperature and when the alloy is heated at a lowertemperature is precipitated in a form which effects hardening of thealloys and improves the creep properties. A heat treatment commonlyemployed comprised subjecting age-hardenable, nickelchromium alloys to ahigh-temperature solution treatment which was usually followed by anageing or precipitation hardening treatment at a lower temperature.While this type of heat treatment usually improved the strengthproperties of such age-hardenable alloys, it was found that theproperties of these heat-treated alloys would vary somewhat at elevatedtemperatures and could not always be obtained consistently. Althoughmany attempts were made to provide heat treatments to overcome theforegoing difficulty and to provide heat-treated alloys having optimumcombination of high-temperature properties, none, as far as we areaware, was entirely satisfactory when carried into practice commerciallyon an industrial scale.

It has now been discovered that an improved combination ofhigh-temperature properties, including long fracture life, can beconsistently obtained by employing a special heat treatment on specialcarbon-containing, agehardenable nickel-chromium alloys andage-hardenable nickel-chromium-cobalt alloys.

It is an object of the present invention to provide a heat treatmentwhich consistently imparts to carboncontaining nickel alloys improvedand optimum strength properties at elevated temperatures.

Another object of the invention is to provide a heat treatment forspecial heat-resistant, age-hardenable, carbon-containing nickel alloys,said heat treatment imparting enhanced strength properties at elevatedtemperatures,

rates Patent "ice particularly improved fracture life, improvedresistance to creep, low creep rate, etc.

It is a further object of the invention to provide improved heat-treatednickel alloys having improved combination of properties at elevatedtemperatures.

Other objects and advantages will become apparent from the followingdescription.

The alloys with which the invention is concerned may be broadly definedas those which contain from 10 to 25% chromium, 1.8 to 4.0% titanium,0.5 to 4.0% aluminium, up to 25% cobalt and up to 10% iron. These alloysalways contain some carbon. The present in vention is based on thediscovery that improved resistance to creep under substantial stress, e.g., 17 long tons per square inch at 750 C., or greater certainty in thedevelopment of the best creep-resisting properties under severeconditions of stress and temperature such as lead to failure in a fewhundred hours, can be produced by certain heat treatments applied toalloys which themselves are characterised by critical carbon contents.

We have discovered that the carbon content should depend upon the cobaltcontent, increasing with the cobalt content.

In the accompanying drawings,

Figure 1 is a graph showing the Way in which the carbon content varieswith and is determined by the cobalt content;

Figures 2 and 3 are graphs showing the variation of the life (on alogarithmic scale) with the temperature employed in one step of the heattreatment; and

Figures 4 to 6 are graphs in each of which temperature is plottedagainst cobalt content, to demonstrate how the temperature in the saidstep may vary with different cobalt contents and corresponding carboncontents in the preferred alloys.

Referring first to Figure 1, we choose an alloy such that the carboncontent is within the range defined by the intersection of the ordinatecorresponding to the cobalt content with the lines A and B. It will beseen that in a cobalt-free alloy the carbon content must be from 0.02 to0.06% and when the cobalt content is 25% the carbon content must be from0.03 to 0.15%.

The heat treatment to which the alloys are subjected according to theinvention consists in first heating the alloy at a temperature of from1150 to 1250 C., the duration of the heating being from 2 to 12 hours at1150" C. and from /2 to 4 hours at 1250 C., with intermediate periods atintermediate temperatures. Next the temperature is reduced to a maximumof 1100 C. and held within the range of 1100 to 800 C. for at least 2hours. At the end of this step the alloy is cooled to room temperatureor other convenient low temperature, and then held at a temperaturewhich leads to substantial hardening, this temperature normally beingwithin the range 650 to 850 C. but being in any case lower than thetemperature of the second step and the alloy being held there for from 1to 24 hours.

Th temperature at which the second step of the heat treatment takesplace depends upon the composition of the alloy, and particularly on thecarbon content. We have found that for any given alloy the time beforerupture when the alloy is tested under severe conditions of stress andtemperature increases to a maximum as the temperature at which thesecond step of the heat treatment is carried out increases, and thendecreases rapidly as this temperature is further increased. Thus we findthat for any given alloy there is a critical temperature, well above thenormal age-hardening temperature, at which the longest life is produced.

The nature of the temperature range, and the way in which it varies withthe carbon and cobalt contents, will be better understood by means ofexamples and by reference to Figures 2 and 3.

Example 1 The alloy from which the results shown in Figure 2 wereobtained contained 0.06% C, 2.33% Ti, 0.99% Al, 19.10% Cr and 0.42% Fe,the balance being substantially all nickel. This alloy, after treatmentfor 3 hours at 1250 C., was cooled directly to 950 C. held there for 24hours, water-quenched and aged for 16 hours at 700 C. As shown in Figure2, the alloy then had a life to rupture of 198 hours under a stress of17 long tons per square inch at 750 C. When the second step of the heattreatment was carried on. at 900 C. instead of 950 C., the life was only100 hours, and when this step was carried on at 1000 C. the life wasonly 120 hours. On the other hand, when the second step was carried onat 1090" C. the life was about 40 hours and when it was carried on at850 C. the life was only 50 hours. In this case, it will be seen, thetemperature range giving substantial life is from about 855 to about1060 C., though to get the best life the second heating step should becarried on in the narrower range of 925 to 1050 C.

Next the results obtained with another'cobalt-free alloy will be given.

Example 2 The alloy in this case contained 0.043% C, 2.36% Ti, 1.13% A1,19.1% Cr and 0.27% Fe, the balance being substantially all nickel. Itthus differed from the alloy of Example 1 principally in its lowercarbon content. When treated and tested as in Example 1 with varioustemperatures in the second step of heat-treatment, the life to rupturewas found to be 221 hours when this temperature was 1025 C., 140 hoursat 1050 C. and only 53 hours at 1075 C.; when the temperature was 950 C.the life was 185 hours, at 850 C. it was 136 hours, at 825 C. it was 90hours and at 800 C. it was only 40 hours. In the case of this alloy,therefore, the second heating step should be carried out in thetemperature range of about 820 to about 1060 C.

The temperature to be used in. the second step of the heat treatmentdepends also on constituents of the alloy other than the carbon. Thus itis materially afiected by the cobalt content. When the cobalt content isas high as 20%, the temperature range for the second step should besomewhat lower.

This. reduction in the temperature used in the second step with a high.cobalt content is illustrated by the fol.- lowing examples.

Example 3 The alloy contained ;044% C, 2.33% Ti, 1.25% A1, 20.2% Cr,19.6% Co, with the balance substantially all nickel. The carbon content,which isthe most important figure other than the cobalt content, was, itwill be seen, substantially the same as in Example 2. The sameheattreatment was applied with varying temperatures in the second step,but this alloy was tested under more severe conditions, i. e., a stressof 19 long tons per square inch at 750 C. When thetemperature in thesecondstep was 950 C. the life was 600 hours. On reducing thetemperature in the second. step to 875 C. the life was 420 hours, to 850C. the life was 280 hours and to 825 C. the life was. 190 hours.Onincre'asing the temperature in the second step to 1025 C. the lifefell to 300 hours and when it was 1050 C. the life was only 140 hours.In the case ofthis alloy, therefore, the range of temperatures in whichthe second heating step shouldbe carried outis from about. 850 to about1025 C. and it is desirable to work ascloselyaspossible to- 950 C.

Example 4 This example, which is illustrated by Figure 3', relatesto analloy containing 20.34% chromium, 19.85 cobalt, 2.44% titanium, 1.13%-aluminium, 0.03% carbon and the balance substantially all nickel, whichwas tested at 19 long tons per square inch and 750 C. With this alloy,when the temperature in the second step was 750 C., the life to rupturewas 7 hours; the life rose to a maximum of 232 hours at 910 C. In thiscase the temperature range giving substantial life is from about 775 to1030 C. and preferably is from 810 to 1010 C.

Figures 4 to 6 show how the ranges of temperature of the second step ofthe heat treatment of the preferred alloys described vary with thecobalt and carbon contents, Figure 4 relating to alloys containing 0.03to 0.05% carbon, Figure 5 to alloys containing 0.06 to 0.09% carbon andFigure 6 to alloys containing 0.10 to 0.15% carbon. In Figure 4 thetemperature must be within the range defined by the intersection of theordinate corresponding to the cobalt content with the lines C and D. Forinstance with 0.04% carbon and 10% cobalt, reference to Figure 4 showsthat the temperature range is from 800 to 1040 C. Likewise in Figure 5the part of the ordinate intersected by the lines E and F, and in Figure6 the part intersected by the lines G and H, shows the range.

We prefer to transfer the alloys straight from the furnace in which thesolution heating takes place to another in which the second heating steptakes place, and to make this :second heating step substantiallyisothermal. However, slow cooling through the permissible temperaturerange of the second heating step may also produce satis factory results.

After the second step of the heat treatment it is usually mostconvenient to let the alloy cool to room temperature and subsequently tore-heat it to the ageing temperature, but if desired the alloy can againbe transferred direct to a furnace in which the ageing takes place, orbe kept in the furnace in which the second step has taken place, thetemperature of that furnace being reduced to the ageing temperature.

The alloys are not restricted in composition to the elements namedabove. As usual, there may be other elements, e. g. silicon up to 1% andcopper up to 2%. Moreover, the alloys may be modified by the addition ofmolybdenum and tungsten up to 10% each, niobium, tantalum or vanadium orany combination of these in a total amount up to 5%, zirconium up to0.3% and boron upto 0.05 In every case the balance of the alloy isnickel, except for impurities and residual deoxidants, e. g. manganese,calcium and magnesium.

While reference has been made throughout to the treatment of the alloys,it is to be understood that in general they are heat treated after beingfabricated into articles or parts (e. g., turbine blades) or into bar,strip, forgings or stampings from which such parts can be machined.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and appended claims.

We claim:

1. A method of improving the resistance to creep of an alloy consistingessentially of from 10 to 25% chromium, 1.8 to 4.0% titanium, 0.5 to 4%aluminium, up to 25% cobalt, up to 10% iron, carbon in an amount withinthe range defined by the intersection of the ordinate corresponding tothe cobalt content with the lines A and B shown in Figure l of theaccompanying drawings, and the balance nickel, except for impurities andresidual deoxidants, which comprises first heating the alloy at atemperature of from 1150 to 1250 C. with the duration of this heatingstep being from. 2 to 12 hours at'1150 C. and from /2 to 4 hours at 1250C. and with intermediate periods at intermediate temperaturessubsequently heating the alloy for at least two hours at a temperaturewhich does not exceed about 1100" C. and is not less than about 800 C.and which is correlated to the carbon and cobalt contents in accordancewith Figs. 4, 5 and 6 of the accompanying drawing, and thereafterholding the alloy for from 1 to 24 hours at a temperature which iswithin the range 650 to 850 C. but is in any case lower than thetemperatures of the preceding heating steps, whereby a heattreated alloyis obtained characterised by having improved fracture life at elevatedtemperatures.

2. A method of improving the resistance to creep of an alloy consistingessentially of from 18 to 20% chromium, 2.0 to 2.4% titanium, 1.0 to1.4% aluminum, up to 25% cobalt, up to 2% iron, carbon in an amountwithin the range defined by the intersection of the ordinatecorresponding to the cobalt content with the lines A and B shown inFigure 1 of the accompanying drawings, and the balance nickel, exceptfor impurities and residual deoxidants, which comprises first heatingthe alloy at a temperature of from 1150 to 1250 C. with the duration ofthis heating step being from 2 to 12 hours when the temperature is at1150 C. and being from /2 to 4 hours when the temperature is at 1250 C.and with the duration of this heating step being for intermediateperiods at intermediate temperatures, subsequently heating the alloy forat least two hours at a temperature which does not exceed about 1100 C.and is not less than about 800 C. and which is correlated to the carbonand cobalt contents in accordance with Figs. 4, 5 and 6 of theaccompanying drawing, and thereafter holding the alloy for from 1 to 24hours at a temperature which is within the range 650 to 850 C. but is inany case lower than the temperature of the second step, whereby aheat-treated alloy is obtained character- 6 ised by having improvedfracture life at elevated temperatures.

3. A method according to claim 2, applied to an alloy with a carboncontent of from 0.03 to 0.05%, in which the temperature in the secondstep of the heat treatment is correlated to the carbon and cobaltcontents in accordance with Figure 4 of the accompanying drawings.

4. A method according to claim 2, applied to an alloy with a carboncontent of from 0.06 to 0.09%, in which the temperature in the secondstep of the heat treatment is correlated to the carbon and cobaltcontents in accordance with Figure 5 of the accompanying drawings.

5. A method according to claim 2, applied to an alloy with a carboncontent of from 0.10 to 0.15%, in Which the temperature in the secondstep of the heat treatment is correlated to the carbon and cobaltcontents in accordance with Figure 6 of the accompanying drawings.

6. The method of improving the resistance to creep of a columbium-freealloy at high temperatures according to claim 1 in which thecolumbium-free alloy being heat treated to impart enhanced fracture lifeat high temperatures contains at least one additional constituent fromthe group consisting of silicon up to 1%, copper up to 2%, molybdenum upto 10% tungsten up to 10%, Zirconium up to 0.3%, and boron up to 0.05%.

2,570,194 Oct. 9, 1951

1. A METHOD OF IMPROVING THE RESISTANCE TO CREEP OF AN ALLOY CONSISTINGESSENTIALLY OF FROM 10 TO 25% CHROMIUM, 1.8 TO 4.0% TITANIUM, 0.5 TO 4%ALUMINIUM, UP TO 25% COBALT, UP TO 10% IRON, CARBON IN AN AMOUNT WITHINTHE RANGE DEFINED BY THE INTERSECTION OF THE ORDINATE CORRESPONDING TOTHE COBALT CONTENT WITH THE LINES A AND B SHOWN IN FIGURE 1 OF THEACCOMPANYING DRAWINGS, AND THE BALANCE NICKEL, EXCEPT FOR IMPURITIES ANDRESIDUAL DEOXIDANTS, WHICH COMPRISES FIRST HEATING THE ALLOY AT ATEMPERATURE OF FROM 1150 TO 1250* C. WITH THE DURATION OF THIS HEATINGSTEP BEING FROM 2 TO 12 HOURS AT 1150* C. AND FROM 1/2 TO 4 HOURS AT1250* C. AND WITH INTERMEDIATE PERIODS AT INTERMEDIATE TEMPERATURESSUBSEQUENTELY HEATING THE ALLOY FOR AT LEAST TWO HOURS AT A TEMPERATUREWHICH DOES NOT EXCEED ABOUT 1100* C. AND IS NOT LESS THAN ABOUT 800* C.AND WHICH IS CORRELATED TO THE CARBON AND COBALT CONTENTS IN ACCORDANCEWITH FIGS. 4, 5 AND 6 OF THE ACCOMPANYING DRAWING, AND THEREAFTERHOLDING THE ALLOY FOR FROM 1 TO 24 HOURS AT A TEMPERATURE WHICH ISWITHIN THE RANGE 650 TO 850* C. BUT IS IN ANY CASE LOWER THAN THETEMPERATURES OF THE PRECEDING HEATING STEPS, WHEREBY A HEATTREATED ALLOYIS OBTAINED CHARACTERISED BY HAVING IMPROVED FRACTURE LIFE AT ELEVATEDTEMPERATURES.